We have proposed that the primate auditory cortex includes a core region of three primary-like areas, surrounded by a belt region containing seven or eight areas, and a parabelt region that has at least two subdivisions. Each region represents a successive level of processing, and subdivisions within each region are activated by parallel inputs. Some aspects of the proposed organization are well-established, while others are not. Thus, one goal is to evaluate and extend the current theory by examining functional organization in the medial belt region, which occupies an intermediate position between the auditory core and insular/parietal cortex, and has not been studied in detail. Characterization of the anatomical and physiological properties of the medial belt fields is important for a more complete understanding of auditory cortical organization, and also in the establishment of the relationship between auditory and somatic sensory systems in cortex. In these experiments, multiunit recordings will be used to guide tracer injections and determine the frequency response area of neuronal clusters within these fields. Through analysis of connections, architecture, and physiology we will evaluate and extend proposals of the organization of medial belt fields, and determine their relationship to auditory-related cortical and subcortical areas. Thus, although the focus of these studies will be on the medial belt region, the experimental design involves analysis at all levels of auditory processing in the cortex, thalamus, and midbrain. A related issue concerns the extent to which the basic principles of auditory cortical organization in monkeys can be generalized to humans. As a second aim, we will compare architectonic features of auditory cortex in non-human primates and humans. Analyses of cytoarchitecture, myeloarchitecture, and the distribution of acetylcholinesterase, cytochrome oxidase, parvalbumin, calbindin, SMI-32, and CAT-301 will be combined to characterize and compare the architecture of core, belt, and parabelt fields in the auditory cortex of monkeys, apes, and humans. We expect to find similarities across species, but important differences may exist that relate to functional specializations. As more is learned concerning the basic functional organization of auditory cortex, it becomes possible to study the effects of pathology on cortical organization and the capacity for recovery.